Generally, the answer is no; it is not possible to stop radioactive decay.
This is because radioactive decay occurs only when it is energetically
favorable and requires no outside input of energy.
There is one type of radioactive decay, electron capture decay, where it
is possible to slow down the rate of decay. Electron capture decay occurs
when an orbital electron is captured by the nucleus resulting in the
effective change of a proton to a neutron. This reduces the Atomic Number
by 1 but does not affect the atomic mass. Because it is possible to change
the amount of energy required to remove an electron, it is possible to
affect the probability of the electron being captured. The largest
difference observed was between an electron in a metal and a fluoride
compound. The difference in the observed half life was approximately 10%.
It would have been much greater at zero degrees Kelvin.
I have just moved, and my reference books are buried in boxes. If you are
interested in reading about the modes of decay and half life, I recommend
Nuclear and Radiochemistry by Friedlander, Kennedy, Macias, and Miller.